1. Field of the Invention
The invention relates to a detecting circuit and a method thereof, and more particularly to a light detecting circuit and a method thereof.
2. Description of Related Art
In the current display market, either the cathode ray tube (CRT) or the liquid crystal display (LCD) can not provide a large frame yet due to the limitation of the process. In addition, for the projector, the large frame is projected on the screen through a light source, e.g. a lamp, and a plurality of lenses. Accordingly, the large frame of 100˜200 inches can be easily projected by the projector in the inner space.
However, in the past, a dark inner space is necessary for the projector because the brightness of the light source in the projector is insufficient. With the progress in science and technology, the light emitting diode (LED) is applied to the projector to solve the issue. The LED has the advantages of high brightness, low power consumption, small volume, long lifespan, and so on. Accordingly, when the LED serves as the light source of the projector, the projector can project a clear frame even if in a bright inner space.
In order to stabilize the brightness of the light beam emitted by the LED, a feedback control mechanism is used when the LED serves as the light source. In the feedback control mechanism, the brightness of the LED is detected, and the current flowing through the LED is adjusted. FIG. 1 is a schematic diagram of a conventional light detecting circuit. Referring to FIG. 1, the light detecting circuit 100 is applied to the feedback control mechanism of the projector. The light detecting circuit 100 includes a light sensor 101, a current mirror 103, and a sensing resistor RSEN. The current mirror 103 is formed by PMOS transistors Q1 and Q2. When being illuminated by the light beam emitted by the LED, the light sensor 101 generates a corresponding photocurrent according to the illumination of the light beam. The current mirror 103 generates a mirror current I flowing through the resistor RSEN according to the photocurrent, so that a voltage drop VSEN across the resistor RSEN is generated. Because the voltage drop VSEN is proportional to the illumination of the light beam emitted by the LED, the illumination thereof can be obtained by measuring the voltage drop VSEN.
Accordingly, when the light sensor 101 senses a light beam with the high illumination, the mirror current I generated by the current mirror 103 and the voltage drop VSEN are both large. Hence, according to the equation VSEN=I×RSEN, the resistance of the resistor RSEN is small because the maximum of the is the voltage VDD. FIG. 2A and FIG. 2B are characteristic illumination-voltage curves of the light detecting circuit in FIG. 1. Referring to FIG. 2A, the resistance of the resistor RB is smaller than that of the resistor RA, so that a larger range of the illumination can be detected while the resistor RB serves as the resistor RSEN. However, if the resistance of the resistor Rsen is small, the voltage interval ΔV corresponding to the illumination interval ΔL is also small, as shown in FIG. 2B. Accordingly, the resolution of the voltage drop Vsen is reduced, and the illumination of the light beam emitted by the LED can not be determined exactly.